Hilarity Ensues because a character must perform an act that is either against their nature or which they have vehemently and constantly protested against doing. Said action can also be just plain humiliating or all of the above—to all parties, perhaps. When this act is aided or witnessed, a character insists that either they must all never speak of this again, ever; or that a specific person who is guaranteed to ride them never hears of it. A common reply to comply with the sentiment is say something to the effect of "Speak about what again?".

This trope can be used to introduce a Noodle Incident, with one character reminding the other they promised never to speak of that again. Alternatively they may have just completed the action, make some passing reference to how unusual or interesting it was, and then pull this line.

Subversions can occur when the character(s) kept in the dark reveal themselves to have known all along or if the character who would have used the incident as ammunition, against all expectations, chooses not to do so. Sometimes that somebody will bring it up again and earning the reaction "I thought we agreed to never speak of it again" or something close to that.

It can be parodied by the person(s) who are told to never mention it again is genuinely unaware of what happened for whatever reason, with them asking "Speak about what again?" getting taken by the person wanting the secret kept as a joke implying compliance with their wish.

Whenever Yumichika has to use his true shikai, which is a secret because he's afraid his macho buddies will disown him, since anyone in that squad whose sword isn't a direct combat type is considered a coward.

Also when Ikkaku uses his Bankai because he's afraid of getting promoted.

In the Yu-Gi-Oh GX dub, Elemental Heroes Sparkman and Avian realize they were brainwashed into loving the Maiden in Love: "Guy pact -- this didn't happen, okay?"

During the first Yu-Gi-Oh, Kaiba sums up the Noah filler arc as a "complete waste of [his] time and effort, so let's move on and pretend that nonsense never happened."

In the end of the first season of Pokémon, Brock decided to stay with a beautiful Professor Ivy. Two seasons later, he returns to Ash's hometown to help his mom and Prof. Oak. Somehow, the relationship ended badly for him, as any mention of her name will cause him to hide under a table or something to that extent. We never learned what happened, and he's not talking. It was never brought up again...

Yugi: Well, it's the end of the movie. All in all, it was kinda disappointing.Téa: I can't help but feel it was all just a cheap cash-in made to promote a bunch of cards.Grampa Moto: It's almost like this whole story was thrown together at the last minute.Pegasus: I agree. So, from now on, let's pretend it never happened.

Freeza from Dragon Ball Z Abridged has one of these after an awkward conversation with Zarbon after the latter mentions his girlfriend.

Freeza: Oh... and all this time I could've sworn you were-- ...never mind.Zarbon: What? You thought I was single?Freeza: Well, no, I just... I thought you were into... you know, it really doesn't matter.Zarbon: Well it matters to me, because, frankly it sounds like you thought I was--Random Minion: Lord Freeza! Vegeta's broken out of the healing--TAAAAANK!(dies by ki blast)Freeza: Oh no, that minion died. Could you go fix that? We'll continue this conversation never.

The Starsky and Hutch film has this in spades. After Starsky (Ben Stiller) and Hutch (Owen Wilson) have to do some fairly embarrassing things to get information out of a con, Hutch starts to talk about it in the car ride back, and Starsky shuts him. "No! No! Let's not talk about it. In fact, let's never speak of this again." Of course, when they get back to the station, it's been recorded and is playing in the Captain's office with all of the other officers watching.

In the film version of The Lord of the Rings: The Two Towers Gimli says the above quote before having Aragorn use him in a Fastball Special (a reference to a scene in the first movie in which he objected to being thrown by Legolas). Aragorn promises not to say a word.

An oblique variant in Spaceballs: after watching the painful footage of being catapulted into the bulkhead of Spaceball One, Dark Helmet says "No, no. Go past this part. In fact never play this part again."

The Hunt for Red October. After Admiral Greer fakes the torpedoing and destruction of the Red October, he tells the torpedo control officer of the frigate he's on:

Admiral Greer: Now, Commander... that torpedo did NOT self-destruct. You heard it hit the hull, and I... was never here.

In the end of Zathura, the sister Lisa says this to her younger brothers, mainly because she thought the future version of one of them was hot before finding out who he was. They bring it up anyway.

Burt Gummer says this after surviving being swallowed by a Grabiod in the third Tremors movie.

National Treasure: Book of Secrets has a part where Gates successfully convinces the president to tell him about the President's Secret Book, in an attempt to find the long-lost City of Gold. However, Gates is warned beforehand:

Ross and Richard are having a sleepover at Rachel and Monica's. At one point they bump into each other in the living room, wearing their girlfriends' bathrobes. After a few awkward seconds they attempt small talk, but then Richard cuts to the chase and says "Let's never speak of this again." Ross says "You got it." and they move on.

Also in "The One Where Joey Loses His Insurance", Joey thanked Chandler for helping him take a shower when he had a hernia, and Chandler freaked out and said, "Now, is that never talking about it again?!"

And there's the time that Monica is stung by a jellyfish. A friend who will pee on you is a friend indeed.

The episode "The Naked Now" features all sorts of weird events when the Enterprise crew falls under the influence of a virus that basically makes everyone more than a little drunk (including Data). One scene features Data meeting up with Tasha Yar in her quarters, where they end up having sex. After the virus has been purged and everyone is sober again, Tasha and Data are back at work on the bridge, where she tells him, "I'm only going to say this once: It never happened."

In "Skin of Evil", Tasha's funeral speech was supposed to include an apologetic "Data, it did happen." It's clear to see where it was supposed to be, as they left in Geordi's reaction shot.

And it's also alluded to in First Contact. Thus, there are some instances where it's treated as if the above scene wasn't cut.

The plot of "Clues" hinges on an instance of this trope. The Enterprise encounters a xenophobic race, the Paxans, who do not tolerate other species' knowledge of them. They incapacitate the crews of ships who come to their home planet, erasing their short-term memories and transporting the ship some distance away, leaving the crew to believe that they have encountered an unstable wormhole. Data, however, cannot be affected, and the Paxans would have destroyed the Enterprise except that Picard ordered him never to reveal what actually happened.

In one episode of Scrubs, Dr. Cox invokes this trope after Dr. Kelso gives an all-too-graphic description of his and Mrs. Kelso's recent sexual encounter.

A vampirized Harmony meets Xander and decides to feed. They apparently regress to their grade school enemy stage (since both of them can fight better), and they're so ineffectual that they eventually just give up. They agree it never happened, and both leave.

Angel played it for laughs in the episode "The Girl in Question". The head of Wolfram and Hart's Rome branch used it several times.

CEO lady(spit) We will speak of it no more!

In an episode of Would I Lie to You?, Neil Morrisey, voice of Bob the Builder, had as one of his "facts" that he once had a builder working at his house who was in fact named Bob. David Mitchell said that if that was true, it was completely unremarkable, and the conversation probably went, "Do you know, I'm actually a builder called Bob." "Right. I do the voice of Bob the Builder." "Right." [short laugh all around] "Let us never speak of this."

The West WingLampshaded its frequent use of of the Walk and Talk by having Sam and Josh walking through the crowded hallways having a conversation as usual. At the end, they both came to a stop and asked each other where they were going. Turns out they were both following each other. Josh: "Let's not tell anyone about this."

Between the Shay siblings, notably, Carly's No Periods, Period in "iDate a Bad Boy" and Spencer's attempt at giving Carly The Talk in "iSpeed Date".

Sam and Freddie's agreement about their First Kiss. When Sam's drugged slip-of-tongue reveals it to Carly, Hilarity Ensues.

Used hilariously in Rome. Cleopatra wants to use The Baby Trap on Caesar, but knowing that Caesar has no sons, (and possibly needing a distraction from her withdrawal symptoms after Going Cold Turkey) she decides to increase her chances of pregnancy by sleeping with one of the legionnaires that Caesar sent to escort her to him. She first orders Vorenus to sleep with her, but Vorenus (despite being sorely tempted) refuses due to fidelity to his wife and Roman sexual attitudes to the idea of being "used" for sex. Vorenus then orders the other legionnaire, Pullo to "Report to Princess Cleopatra and do whatever she tells you to." Cue a prolonged and noisy bout of sex, much to the discomfort of Vorenus. When Pullo returns, they have the following conversation:

Pullo: Gods, that was something, let me tell you.Vorenus: I don't want to hear about it. If you're wise, you'll never speak of this again.Pullo: Why? I was only obeying orders. Bloody good orders, too.

Invoked almost verbatim by Cristina in the Greys Anatomy episode "Complications of the Heart", after a surgery wherein both she and the more experienced Dr. Altman forgot how to perform a very basic procedure and ended up needing the nurse to bail them out.

Almost verbatim on The Peter Serafinowicz Show. Upon making a breakthrough deduction in a case, Sherlock Holmes becomes so sexually aroused that he forces himself on Watson. In post-coital reflection, Holmes says "We shall not speak of this again."

One happens offscreen in Black Books, we only see Bernard's reaction when Manny breaks silence.

Manny: Yes, it'll be quite some time before I want to sacrifice another monkey.(Bernard lunges forward and grabs him)Bernard: YOU SAID WE WOULDN'T TALK ABOUT CANADA!

There's a Mental Phlebotinum Breakdown in Sasha's mind, and a horrible mutated monster comes up and proceeds to pound the living daylights out of Sasha in a highly embarrassing fashion. Raz saves Sasha's life, for which Sasha grants him his merit badge. This conversation then occurs:

Raz: So, is this the part where I get a speech and learn another lesson...?Sasha: No. Here is your merit badge. Let us never speak of this again.

If you return to clean up loose ends, Raz will bring it up again. Sasha will reveal that he expected Raz to overload the machine, and the course he ran was equally planned, to allow Raz to learn to use his power in practical application. The Megacensor was where things got out of hand, though.

From Dragon Age, Sten says this to Leliana when she sees him playing with a kitten and picking flowers. Also comes up after the threesome/foursome with Isabela in The Pearl.

Zevran: I don't mind never speaking of it again, as long as we could do it again.

In Cave Story, one of your fights with Balrog begins after you almost trick him into helping you with a feat that requires great strength. When you win, he yields and assists you (this is one of the hints dropped to you that he isn’t really such a bad fellow; he's just doing his job). He entreats you, however, not to tell anyone he did it.

Abe Lincoln must die! has Sam and Max take a date test to find the perfect match for each other. The result gives... Sam to Max and Max to Sam. Both agree never to speak about it again.

One of the categories for the third annual Neopies on Neopets is named after this trope.

Trucy: I can still remember that moment... You brandished those bloomers on high, and shouted... "Objection!"Apollo: ...Here's what I want you to do, Trucy. Take that memory, gently lock it away deep in your heart, and never speak of it.

You may say, "But I don't HAVE any enemies!" Well, do you have FRIENDS? Are they on FIRE? Right NOW? Jarate them to a non-on fire state INSTANTLY! Being on fire is horrible! They'll never want to talk about your Jarate rescue ever again, but you'll have their secret undying gratitude!

While being interviewed for the Lawful Good afterlife, hero Roy Greenhilt stops the deva interviewing him before she can mention a Gender Bender incident within earshot of his father, who is standing not ten feet away.

During that very incident, Roy also insisted Belkar not know. Subverted in that Belkar knew it was Roy all along and was comfortable enough with his masculinity to hit on Roy purely to mess with him.

And during Azure City's New Year, Belkar gets drunk and kisses Vaarsuvius full on the lips. Afterwards, Belkar forgets it entirely and Vaarsuvius insists that, if the one witness to the kiss (Durkon) ever has to discuss it again, it will be referred to it as "the Event". And they never talk about it again.

When Vaarsuvius is rescued from the Para-Elemental Plane of Ranch Dressing, he/she mentions that there were some adventures before they got there.

Belkar: Maybe we'll see that as a bonus story in one of the books!V: Maybe we shall never mention it again.Belkar: Yeah, probably that.

Crystal: My head is killing me. Does anyone remember what happened after the tequilla shots?Riff: Let's see. We're charred, we have twigs in our hair and we're covered in ducks.Bun-bun: I vote we all go home and never speak of this again.

Bard: Vas uses his change of gender to quell the rage of one of the more violent patrons by... stuffing her face into his new rack. It takes a bit for this to register with him, though.

In Questionable Content#447, Marten and Marten's mother, who is a professional dominatrix return to his apartment to find Pintsize wrapped up in bondage gear, which prompts this line to be spoken.

In Homestuck, Karkat takes this to the logical extreme by trolling John backwards, so that they'll never be able to talk about the first conversation (from Karkat's perspective), where Karkat accidentally reveals his hatecrush for John. Needless to say, it's hilarious.

In Housepets, Peanut Butter and Grape happen to see Fido and Sabrina do... something, in this comic. Afterward, Grape tells Peanut,[1] "We never speak of this again."

South Park: In the "Fantastic Easter Special" episode, Jewish Kyle and Jesus Christ are locked in a cell. Jesus can escape to stop the episode's villain, but only if Kyle kills him so he can resurrect outside the building. Kyle tells Jesus before reluctantly doing the deed "Eric Cartman can never find out about this."

Marge also said it in regard to a relative who snapped and went on a shooting spree:

Marge: Well, Bart, your Uncle Arthur used to have a saying: "Shoot 'em all and let God sort 'em out". Unfortunately, one day he put his theory into practice. It took 75 Federal Marshalls to bring him down. Now let's never speak of him again.

Subverted, since after saying this line she immediately references the incident again:

Marge: Honey, you should listen to your heart and not the voices in your head -- like a certain uncle did one grey December morn...

This also happened after the episode where Principal Skinner was revealed to be really an imposter named Armin Tamzarian. The cleared up the problem by deciding they were happier with the fake Skinner, and the real one was run out of town. The local judge issues a decree that no-one will ever mention it again, under penalty of torture. And, this being the Simpsons, they do mention it again—when Skinner is giving Lisa a hard time about naming a new cat of hers the same as her old cat, she calls him Mr. Tamzarian.

Another episode introduces Grandpa's brother Cyrus, with dialogue to this effect (paraphrasing):

Bart & Lisa: You never told us you had a brother, grandpa.Grandpa: Yes, and I'll never speak of him again.

The Powerpuff Girls had episode "Shotgun Wedding", in which the Professor tries to learn more about Fuzzy Lumpkins for the purpose of science, and after getting covered in mud, flowers, and a curtain is mistaken for a woman by Fuzzy and nearly forced to marry him. After the Professor is saved by the Powerpuff Girls, he writes on notepad that he was using "Lumpkins are, above all, stupid" and as a sidenote "never speak of this again"... and then eats it. He gets really mad when the Girls mention it about 10 seconds later.

Grim: We keep flying, we never head back, and we never talk about this again.

The 2008 BotCon script reading, "'Bee in the City", ends with all parties involved agreeing to never speak of the incident again.

In Hey Arnold!, whenever Phoebe ends up involved on something involving Helga's crush on Arnold, Helga always tells her this at the end.

Helga: This whole thing? It never happened.Phoebe: Forgetting!

Also:

Principal Wartz:(in a sombreo shaking maracas) Are you attempting to leave the school grounds without permission, Miss Pataki?Helga: Are you attempting to dance the flamenco, Principal Wartz?Principal Wartz:(beat) This moment never happened.Helga: Gotcha.

In Recess all the group agreed this when they see Spinelli having fun dancing Hula Hula with Finster, who was babysitting her in that episode.

In the Justice League episode "This Little Piggy" we get to see how far Batman will go to save Wonder Woman here.

In one crossover between Teenage Mutant Ninja Turtles and Usagi Yojimbo, Usagi says this when he and Leonardo put on black cloaks and infiltrate a fortress. While Usagi respects the Turtles, he doesn't really like ninjas and their "cowardly and dishonorable" ways.

In The Penguins of Madagascar, ("Jiggles" episode), after Kowalski's Blob Monster spat both Kowalski and Julien out, the two were covered in green goo and draped across each other. Julien's response to the situation is, "I will not talk of this if you don't."

Ikazuki: Well? What are you waiting for?!Ratso: Uh, well, this is kinda awkward.Ikazuki: Simpleton! Do as you are told!Finn: What he said.Ratso: Okay! But we are never gonna bring this up again.Chow: I'm not even here.

Danny Phantom: In "Reality Trip", Sam wonders how Danny convinced her parents to let her go on a cross-country trip with him and Tucker, and the explanation is thus:

Sam's Mother:(being overshadowed by Danny) You know I think the Fentons are wonderful, Sam should take a cross-contry trip with them.(Danny pops out of her, and overshadows Sam's father)Sam's Father: And I agree with you! Let's never speak of this again.

In Wakfu season 2 episode 5, "The Dragon-Pig", for not listening to Sadlygrove's warning, Evangelyne, Amalia and Yugo fall victims to a Baleful Polymorphturning them into cute, but defenseless piglets. Once they regain human form, Amalia insists that this incident should never be mentioned again.

In the All Just a Dream episode of Kim Possible, Kim saves villains Drakken and Shego from an exploding device. After Shego realizes how awkward it is, Drakken declares this trope, to which Kim replies "Fair enough".

On Jimmy Two-Shoes, after an episode ends on a particularly disgusting note, Jimmy and Beezy agree "This never happened."

My Little Pony Friendship Is Magic, "The Return of Harmony, Part 2": Rarity says this word for word to her friends about the fact that she was brainwashed into thinking a large boulder was a giant diamond. Of course, Applejack brings it up very shortly afterwards.

Rarity: How in Equestria could [Rainbow Dash] think that tiny patch of cloud is Cloudsdale?Applejack: The same way [Discord] got you to think that cheap rock was a bonafide diamond?Rarity: ...I thought we agreed never to speak of that again.

King of the Hill: Years ago, Hank, Dale and Bill "borrowed" Boomhauer's car for a joyride and droven it into a flooded quarry. The three then agreed never to mention it again. Hank had no choice but to break the agreement when the quarry was about to be drained, since they left clues linking them to the case.

In the Grand Finale of Gravity Falls, the town actually passes a law called the "Never Mind All That" Act, which forbids any town resident from talking about Weirdmageddon, and this was actually one of the few unique laws of the town that makes sense.